1. Field of the Invention
The present invention relates to an analog-digital converter.
2. Description of the Related Art
As is well known, an analog-digital converter requires a clock signal for carrying out the quantization of the samples of an analog signal to be converted and performing other timing functions. By way of example, let us consider an analog digital converter of the SAR type (Successive Approximation Register) of which the basic structure is as shown in FIG. 1. It comprises an analog-digital converter DAC (which includes a reference voltage source VREF) and a comparator COMP as elements for the quantization of an input analog quantity to be converted. It also comprises a logic unit LOG (which includes a register REG) with timing and control functions connected to the converter DAC and the comparator COMP by means of a control bus CTRLBUS. A timing or clock pulse signal (CLOCK) is applied to a terminal CK of the logic unit. An input voltage to be converted VIN is applied to the converter DAC and a conversion request signal CONVREQ is applied to a starting terminal STR of the logic unit LOG. A sequence of clock-pulse timed signals activates the converter DAC and the comparator COMP and makes possible the loading of the sample VIN to be converted. The register REG is set to a digital value corresponding to the centre of the scale of the conversion range (10000000 in the case of an 8-bit register). The converter DAC therefore furnishes an output voltage of VDAC=VREF/2. The comparator COMP compares this voltage with the voltage VIN and provides the result of the comparison to the logic unit. When VIN>VDAC, the comparator output is a logic “1” and the most significant bit of the register remains at “1”. But when VIN<VDAC, the output of the comparator is a logic “0” and the most significant bit of the register is switched into a logic “0”. The next bit of the register is then set to “1”, followed by another comparison with the same criterion, and this is continued until the least significant bit has been examined. At this point the conversion is finished and the register contains the digital code, in this example consisting of 8 bits, corresponding to the input voltage VIN. This code is available at the output OUTBUS of the logic unit LOG and the register REG, which is the output of the converter.
A typical field of application of analog-digital converters is that of cellular telephones. These telephones call for the presence of an analog-digital converter to convert analog signals of various types needed for such control functions as measuring the charging level of the battery, measuring the charging current of the battery, measuring the internal temperature, verifying the presence and the type of accessories connected to the cellular phone, and so on. The number of inputs to be converted is typically of the order of 10–15.
With a converter of this type it is necessary for the sample of the analog signal to be converted to be memorized even in the absence of the conversion clock signal, which is normally obtained from the system clock. The memorization request occurs typically during the charging of the battery while the telephone is switched off and is based on the use of the oscillator that controls the telephone's clock (understood as device for measuring time), but the oscillator frequency is too low to permit its being used as clock signal of the conversion. A known solution, illustrated by FIG. 2, envisages applying the analog voltage VIN to be converted onto a capacitor C by means of a signal STORE supplied by a logic controlled by the oscillator and waiting until the system clock signal becomes available again for activating both the conversion clock and the conversion request. Since the input of the converter DAC is subject to losses, maintenance of the voltage VIN for the whole of the necessary time calls for the use of a relatively large capacitor C that can be integrated only at the price of occupying a very large area of the integrated circuit.
This type of application calls for the lowest possible energy consumption not only during the conversion operations, but also and above all in stand-by conditions.